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What is Telecentricity?
By Jim Michalski
Telecentricity is a special property of certain multi-element lens designs in which the chief
rays for all points across the object or image are collimated. For example, telecentricity
occurs when the chief rays are parallel to the optical axis, in object and/or image space.
Another way of describing telecentricity is to state that the entrance pupil and/or exit pupil
of the system is located at infinity (see Figures A and B).
Figure A
Telecentric lens drawing showing discrete object points being imaged.
Figure B
Telecentric lens drawing showing chief rays for each object point. Note that the chief rays
are parallel to the optical axis.
ADVANTAGES
This definition of
telecentricity leads to a
number of questions. Why
is telecentricity desirable?
What are its advantages,
disadvantages, and
limitations? For many
applications, telecentricity
is desirable because it
provides nearly constant
magnification over a range
of working distances,
virtually eliminating
perspective angle error.
This means that object
movement does not affect
image magnification.
In a system with object
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space telecentricity,
movement of the object
toward or away from the
lens will not result in the
image getting bigger or
smaller, and an object
which has depth or extent
along the optical axis will
not appear as if it is tilted.
For example, a cylindrical
object whose cylindrical
axis is parallel to the
optical axis will appear to
be circular in the image
plane of a telecentric lens.
In a non-telecentric lens
this same object will look
like the Leaning Tower of
Pisa; the top of the object
will appear to be elliptical,
not circular, and the side
walls will be visible (see
Figure C).
Figure C:
Non-telecentric, Telecentric
Imaging results at the image plane of a telecentric and nontelecentric lens system. Notice the telecentric system
eliminates perspective distortion.
In systems with image
space telecentricity,
image plane movements
to focus or intentionally
defocus the system will
not change the image size.
This property is
fundamental to today's
microlithography industry
where tolerances on
feature size are routinely
below a tenth of a micron.
An additional advantage of
image space telecentricity
is that it can lead to
extremely uniform image
plane illumination. The
4
normal cos q falloff in
image plane illumination
from the optical axis to the
edge of the field is
removed, since all chief
rays have an angle of θ °
with respect to the image
plane.
DISADVANTAGES
There are a number of qualities inherent in telecentric lenses which may be considered
disadvantages. First, the optical elements in the region of telecentricity (image side or
object side) tend to grow in size. In the case of a doubly telecentric design (telecentric in
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both object and image space), both the front and rearmost lens groups need to be bigger
than the object and image respectively. Thus, a one hundred millimeter object or image
height will require a lens of significantly larger aperture diameter to provide an
unvignetted field of view along with mechanical mounting and retention features. This is
illustrated in Diagram 1 and 3 (see below).
The graphics below (Figures 1-3) depict examples of both telecentric and nontelecentric lenses designed by Edmund Optics. These examples include 0.5X relay
lenses (both telecentric and non-telecentric) along with a doubly telecentric 0.25X
reduction camera lens for microlithographic use operating at the ArF laser line (0.193
microns).
Diagram 1
Y-Z profile drawing of a 0.5X telecentric lens (telecentric on object side)
Diagram 2
Y-Z profile drawing of a 0.5X non-telecentric lens
Diagram 3
Doubly telecentric 0.25X reduction camera lens for microlithographic use operating at
the ArF laser line (0.193 microns)
Diagram 1 is a Y-Z profile drawing of a 0.5X telecentric lens and Diagram 2 is a Y-Z profile
drawing of a 0.5X non-telecentric lens. It is important to note that both lenses have the
same lateral magnification (0.5X), object working distance (125mm), object height (semidiameter = 11.0mm), and the same working F/# (F/6). The front lens group of the
telecentric lens has a diameter of approximately 40mm while that of the non-telecentric
lens has a diameter of less than 20mm - a factor of more than two times smaller.
A second disadvantage of telecentric designs is that they tend to be more complex than
non-telecentric designs. This is illustrated by the difference in element count within the
two designs shown in Diagram 1 and 2. The telecentric design has seven elements while
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the non-telecentric design is comprised of only five. It should be pointed out that the nontelecentric lens in Diagram 2 can easily be made to perform at F/6 (it was originally
designed to work at F/2) with only four elements, three less than the telecentric lens.
As with many other subjects, there is a common misconception concerning depth of field
and telecentricity. The misconception about telecentric lenses is that they have a larger
depth of field than ordinary lenses.
Realistically, telecentricity does not imply large depth of field, which is only dependent on
F-number and resolution. With telecentric lenses, objects still blur farther away from best
focus, but they blur symmetrically, which can be used to one's advantage. As long as the
object's features are within the telecentric working distance, the magnification will not
change. In other words, features closer to the lens do not appear larger than those
furthest away.
APPLICATION USES
There are a variety of applications that depend upon, or can benefit from, the use of
telecentric optics. These applications include CCD based measurement systems, metrology
equipment, and microlithographic camera systems.
CCD based measurement systems can be used to measure the spacing and/or size of a
number of objects on an electrical or mechanical component. The precise measurements of
objects or features, or their separations, is accomplished through the use of measurement
software. This type of software uses centroiding algorithms in the calculations of object
separation and size. A telecentric lens is ideal for this application because extended objects
will appear symmetrical, whereas the image from a non-telecentric optic will be elliptical
(see Figure D). The improvement in measurement accuracy resulting from the use of a
telecentric lens can often mean the difference between success or failure.
Figure D
Telecentric, Non-telecentric
Extended objects will appear symmetrical in a telecentric optic, whereas the image from a
non-telecentric optic will be elliptical.
Many metrology systems also depend upon telecentric optics. A profile projector is one
example of such a system. The profile projector is used to measure an object, or a feature
within an object by projecting an image of the area under test onto a screen. This
projected image is then either compared to a "gold standard" reference at the proper
magnification, or it is measured directly and its dimensions compared to the nominal
designed part. This type of measurement absolutely requires that magnification does not
change with object position. If not telecentric, this type of instrument would give a
different measurement result each time the working distance to the object was changed.
This is obviously not desirable in an instrument designed to provide absolute
measurements.
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Microlithographic camera systems also depend upon the property of telecentricity in their
application. These lens systems are often doubly telecentric-telecentric in both image and
object space. These lens systems, which can cost in excess of hundreds of thousands of
dollars, are used in the manufacture of integrated circuits (IC's). The wafers or chips upon
which the IC's are fabricated go through many imaging operations, which create the
surface features on the wafer. These features are routinely sub-micron in size and getting
smaller with every new generation of microlithographic equipment. The size of these
features, along with their absolute locations, must be controlled to small fractions of a
micron. This problem is exacerbated by the overlay necessary when numerous resist
exposures and etches are required in the IC production process. Maintaining a constant
magnification through the use of telecentric optics is crucial in this whole process.
SUMMARY
In summary, this article has defined the difference between telecentric and non-telecentric
optics. It has also presented some of the benefits, liabilities and limitations of telecentric
optical systems. In addition, a number of applications and sample designs have been
presented. If you have any questions or a need for further discussion, please feel free to
contact our Applications Engineering department at (800) 363-1992 or
techsup@edmundoptics.com.
Jim Michalski joined Edmund Optics in May 1999 as our Chief Scientist
after working in the defense industry for 13 years. He has a BS in Optical
Engineering from the University of Rochester and is working on his MS in
Optical Engineering from the University of Arizona. Jim has 20 years of
lens design experience working in the UV, visible, NIR, 3-5 micron and 712 micron spectral ranges.
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